1. Field of the Invention
The present invention relates to propylene polymers. In particular, the present invention concerns efficiently nucleated propylene homo- and copolymers. The invention further concerns a process for preparing such compositions. Finally, the present invention relates to the use of the new propylene polymers for the manufacture of products by, e.g., extrusion, blow moulding, thermoforming and injection moulding. Examples of such articles are tubes, pipes and fittings, housings of various appliances and cups and pails.
2. Description of Related Art
Propylene (PP) homo- and copolymers have excellent resistance to heat and chemicals as well as attractive mechanical properties. These characteristics make propylene polymers more suitable than polyethylene for some uses, for example in pipes, fittings and other similar articles formed by extrusion or moulding. However, processing of polypropylene by, e.g., injection moulding, thermoforming or blow moulding, to form thin-walled containers has resulted in products having insufficient stiffness, transparency and cycle time. This is caused by the semi-crystalline nature of polypropylene. Injection moulding processing has also resulted in products having insufficient stiffness and cycle time.
In the prior art it has been proposed to improve the transparency of moulded polypropylene by blending the polymer with various nucleating agents such as dibenzilidene sorbitol (DBS), sodium benzoate or di(alkylbenzilidene)sorbitol. These traditional nucleating agents tend to bleed out from the polymer composition during processing and many of them give rise to fumes with an offensive smell. As a solution to these problems, it has been suggested in the art to use vinyl compounds, such as polymers of vinyl cycloalkanes and 3-methyl-1-butene, as nucleating agents in the form of propylene copolymers or polypropylene compounds, cf. EP Patent Specifications Nos. 0 151 883, 0 152 701, 0 206 515, 0 368 577 0 369 658 and 0 417 319. However, the known polymerically nucleated polypropylenes still have somewhat low isotacticity and the yield and productivity of the known polymerization processes are not satisfactory. Further, there is no suggestion in the afore-mentioned EP Patents that these polymers would be suitable for manufacture of pipes and fittings.
It is an object of the present invention to eliminate the problems related to the prior art and to provide a novel high crystallinity propylene polymer having high isotacticity and excellent mechanical properties.
A second object of the invention is to provide a novel process with high productivity for preparing novel propylene polymer compositions of the above kind.
It is still a further object of the present invention to provide extruded and moulded products comprising high crystallinity propylene polymer compositions.
These and other objects, together with the advantages thereof over known processes and products, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
The invention is based on the idea of providing a propylene polymer which is nucleated with 0.0001 to 1% by weight of a polymerized vinyl compound and containing less than 0.01 wt-ppm (or below the limit of detection of the Gas Chromatography-Mass Spectrometry, GC-MS, method) of unreacted monomeric vinyl compounds. Polymerization of propylene optionally with comonomers in the presence of a transesterified Ziegler-Natta catalyst system comprising a strongly coordinating external donor will yield a nucleated polymer of the above kind having improved isotacticity. Thus, homopolymers prepared with a ZN catalyst system modified with a polymerized vinyl compound will have a content of less than 2.5%, in particular less than 2% xylene solubles at 23xc2x0 C., a crystallization temperature of over 124xc2x0 C., in particular 126xc2x0 C. or higher, and a tensile modulus of greater than 2,000 MPa, preferably greater than 2,100 MPa (or even greater than 2,200 MPa). By using a modified catalyst composition containing practically no or only minute amounts (preferably less than 2000 ppm, in particular less than 1000 ppm of monomer residues ) for the manufacture of the propylene polymer, no separate washing steps are needed and high catalyst activity can be maintained.
Finally, it has now been observed that high polymerization temperature increases isotacticity. Thus, the amount of xylene solubles decreases with 20 to 25% or more when the polymerization temperature is increased from 70xc2x0 C. to 90xc2x0 C.
The efficiently nucleated propylene polymers are particularly suitable for use tubes, pipes and fittings, as well as in buffer tubes of optical cables.
More specifically, the polymer according to the present invention is characterized by what is stated in the characterizing part of claim 1.
The process according to the present invention for preparing nucleated polypropylene compositions is characterized by what is stated in the characterizing part of claim 6.
The present polymer articles are characterized by what is stated in the characterizing parts of claims 34 and 42.
The invention achieves a number of considerable advantages, some of which were already discussed above. In particular it can be noted that the present high-crystallinity propylene polymers are characterized by high crystallinity and high crystallization temperature. In comparison to conventional polypropylene the present polymers exhibit good mechanical properties, such as high modulus, high heat resistance and water vapour barrier. Very good and consistant nucleation improves clarity in a better way than with conventional nucleating agents. Nucleation dominates effect from different pigments; this means consistent shrinkage and warpage in multicoloured parts. The crystallinity is influenced by the high isotacticity (preferably greater than 98%) of the polymer and by the effective nucleation with the polymerised vinyl compounds.
With the aid of high polymerization temperatures, the activity of the catalyst is increased by about 80%. Thus, to mention an example, by using a transesterified MgCl2 supported TiCl4 catalyst prepared according to FI 88047 and dicyclopentyl dimethdxy silane (also known as donor D) as an external electron donor (in the following this catalyst is also abbreviated BC-1/D), after a one hour polymerization, the activity of the catalyst presently used at a polymerization temperature of 90xc2x0 C. was about 80 kg/g cat, whereas at 70xc2x0 C. the activity of the same catalyst was less than 45 kg/g cat.
The present compositions can be used in any kind of polymer articles. Particular advantages are obtained by applying the compositions to the manufacture of appliances, automotive parts, cups, pails, containers, caps or closures. The new material can also be used in various cable, tube and pipe applications. These are, for example, fiber optic buffer tubes, smooth solid wall pipes, fittings, and pipe system details, e.g. valves, chambers and manholes, for indoor or buried sewage, multilayer pipes and fittings for indoor or buried sewages, and structured wall pipes and fittings for buried sewage.
The material used according to the present invention will in a cost effective way give pipes with clearly higher stiffness than standard heterophasic copolymers, measured on plaques of the material itself or on pipes, and improved or retained impact properties. The combination of high HDT and high. stiffness means that it is possible to reduce wall thickness and by this optimize cycle time. Compared to mineral filled PP, the present products provide low cost due to reduced volume price (lower density). Better scratch resistance and high gloss is also obtained. The stiffness of polypropylene pipes is increased not only in radial direction but also in axial direction. Furthermore, the pressure resistance (Slow Crack Growth Properties) is improved compared to standard polypropylene which leads to better long term properties.
Not only do the present propylene polymers render the products excellent mechanical properties, some of which are discussed above, but preliminary trials indicate that they also improve pipe extrusion by yielding higher output and better pipe surfaces than conventional polypropylene having a similar molar mass distribution (MWD) of homopolymer phase and rubber phase. In connection with injection moulding, faster cycle times for fittings can be obtained.
A further important advantage of the invention resides in the fact that the present polymers will achieve low-cost formulations, which makes it possible to reach about the same final cost per length of pipe as with PVC. In particular, by adding small amounts of talc the stiffness of the polypropylene compositions can be further improved which reduces PP raw material costs of the final products.
Next, the invention will be more closely examined with the aid of the following detailed description.